Parts of a Computer:
(i) Input Unit
(ii) Central Processing Unit (CPU)
(iii) Output Unit

The eye has three main parts namely cornea, iris and pupil

Digestion and absorption of food done by digestive system.

Cell wall and chloroplast found only in plant cell

Robert hook discovered the cell in 1665.

Our earth surrounded by a huge envelope of air is called 'Atmosphere'. Five layers of atmosphere are:
(i) Troposphere
(ii) Stratosphere
(iii) Mesosphere
(iv) Ionosphere
(v) Exosphere

If we remove the fish from the aquarium and keep it in a dark place with green plants,
(i) they can't have sunlight and
(ii) have no CO₂ from the water produced by fish as they breathe.
(iii) Simply say, there is no possibility for photosynthesis to make their food.

When solute is dissolved in a solvent it forms a solution.
Solute + Solvent ⟶ Solution

Yes, we can make a candle doll from candle wax. It is a kind of physical change.

Simple circuit is there in a torch light.

(i) The measurement of warmness or coldness of a substance is known as its Temperature.
(ii) SI unit of temperature is kelvin

(i) The Water heater
(ii) Iron box
(iii) Electric kettle etc.

PHOTOSYNTHESIS:
Plants manufacture food by a process called photosynthesis. During photosynthesis, carbon-dioxide from the air and water from the soil react in the presence of sunlight and chlorophyll to produce food.
\(\text { Carbon-di-oxide }+\text { water } \underset{\text { Chlorophyll }}{\stackrel{\text { Sunlight }}{\longrightarrow}} \text { food }+\text { oxygen }\) 
RESPIRATIO IN ANIMAL :
When we breathe in air, the oxygen present in the air reacts chemically with digested food within the body to produce carbon-di-oxide gas, water vapour and energy.
Food + Oxygen  Carbon-di-oxide + Water + Energy
(i) carbon-di-oxide formed during respiration dissolves in the blood and is exhaled out of the body through the lungs.
(ii) Plants release oxygen during photosynthesis which is much more than the oxygen consumed by the plants, during respiration.
(iii) Thus, the balance of O₂ & CO₂ in air is maintained.

Conductors:
(i) The rate of flow of electric charges in a circuit is called electric current.
(ii) The materials which allow electric charges to pass through them are called conductors.
(iii) Examples: Copper, iron, aluminium, impure water, eafth etc.
Insulators (Non -Conductors) : 
(i) The material which do not allow electric charges to pass through them are called insulators or non- conductors.
(ii) Examples: plastic, glass, wood, rubber, chinna clav, ebonite etc.

The expansion of substance on heating is called, the thermal expansion of that substance 
(i) Gaps are left in between rails while laying railway track :
So that there is enough margin for the tracks to expand and contract due to the temperature changes. If that gap is not left then the tracks might have enormous stress in them while expanding due to heat, which can damage them.
(ii) Gaps are left in between two joints of a concrete bridge:
Thermal expansion takes place due to the increase in the kinetic energy of the constituent molecules as they are heated. Thermal expansion is the reason we leave some gaps between section of h concrete bridge.

While swallowing food if the food particles enter into respiratory tract leads to hiccup and cough to eject out the food particles from respiratory tract.

Thick walls are present in ventricles to pump the blood from the heart to various part of body.

a) melting of wax and
d) Solidification of the molten wax can be reversed.
Burning of candle is a physical / reversible change. That is,
\(\text { Solid wax } \underset{\text { Cooling }}{\stackrel{\text { Heating }}{\rightleftarrows}} \text { molten wax }\)

Both A and R are correct and R is the correct explanation for A.